Superconducting magnesium diboride thin film and method and apparatus for fabricating the same

ABSTRACT

A superconducting magnesium diboride (MgB 2 ) thin film having c-axial orientation and a method and apparatus for fabricating the same are provided. The fabrication method includes forming a boron thin film on a substrate and thermally processing the substrate on which the boron thin film is formed along with a magnesium source and cooling the resulting structure. The superconducting magnesium diboride thin film can be used in a variety of electronic devices employing superconducting thin films, such as precision medical diagnosis equipment using superconducting quantum interface devices (SQUIDs) capable of sensing weak magnetic fields, microwave communications equipment used for satellite communications, and Josephson devices. Computer systems with 100 times greater computing speed can be implemented with the superconducting magnesium diboride thin film.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and apparatus forfabricating a superconducting magnesium diboride (MgB₂), and moreparticularly, to a superconducting magnesium diboride thin film havingc-axial orientation and high temperature superconductivity, and methodand apparatus for fabricating the superconducting magnesium diboridethin film.

[0003] 2. Description of the Related Art

[0004] Recently, a research report on superconductivity in magnesiumdiboride (MaB₂) in Nature 410, p.63, Mar. 1, 2001 by Nagamatsu et al.discloses superconducting magnesium diboride having a transitiontemperature as high as 39 K, compared to the transition temperature of23 K for conventional superconducting metals. The magnesium diboridealso has high current transporting capability due to higherconduction-electron density Thus, it is highly probable that almost allexisting conventional superconducting materials will be replaced withthe magnesium diboride superconductor.

[0005] Such highly probable applicability of the superconductingmagnesium diboride has boosted recent research on superconductingmagnesium diboride worldwide. As an example, Canfield et al. at the IowaState Univ. in the U.S. developed superconducting wires for practicaluses (Phys. Rev., Lett. 86, 2423 (2001)).

[0006] In addition, processing of superconducting magnesium diborideinto a thin film is essential for its application in a variety ofelectronic devices. However, there have not yet been any reports ofsuperconducting magnesium diboride in the form of thin film withsatisfactory effects.

SUMMARY OF THE INVENTION

[0007] It is a first object of the present invention to provide a methodfor fabricating a superconducting magnesium diboride (MgB₂) thin film.

[0008] It is a second object of the present invention to provide asuperconducting magnesium diboride thin film formed by the method.

[0009] It is a third object of the present invention to provide anapparatus for fabricating a superconducting magnesium diboride thinfilm.

[0010] To achieve the first object of the present invention, there isprovided a method for forming a superconducting magnesium diboride(MgB₂) thin film, the method comprising: (a) forming a boron thin filmon a substrate; and (b) thermally processing the substrate on which theboron thin film is formed along with a magnesium source and cooling theresulting structure.

[0011] It is preferable that, in step (a), the boron thin film is formedby pulsed laser deposition, sputtering deposition, electron beamevaporation, metallorganic chemical vapor deposition, or chemical vapordeposition.

[0012] It is preferable that, in step (b), the substrate with the boronthin film and the magnesium source are heated at a temperature of600-1000° C. in the absence of any reactive gas such as air.

[0013] It is preferable that step (b) is carried out in a state wherethe substrate with the boron thin film and the magnesium source aredouble sealed with a container made of tantalum or niobium inside and acontainer made of quartz outside. As a result, a magnesium diboride thinfilm having good superconductivity can be obtained.

[0014] It is preferable that both ends of the container made of tantalumor niobium are sealed in an inert gas atmosphere, and both ends of thecontainer made of quartz are sealed in a vacuum. In step (b), thetemperature of the heat source is raised to 600-1000° C., and thesubstrate with the boron thin film and the magnesium source are placedinside the heat source, rapidly heated at the temperature of 600-1000°C. for 10-60 minutes, and cooled in the heat source to room temperature.

[0015] The second object of the present invention is achieved by asuperconducting magnesium diboride thin film formed by the method ofclaim 1 with the c-axial crystal orientation.

[0016] The third object of the present invention is achieved by anapparatus for fabricating a superconducting magnesium diboride thinfilm, the apparatus comprising: a first protecting member receiving asubstrate with a magnesium diboride thin film and a magnesium source forpreventing the magnesium diboride thin film and the magnesium sourcefrom oxidizing in contact with the air; a second protecting memberreceiving the first protecting member for preventing oxidization of thefirst protecting member; and a heat source for thermally processing thesubstrate with the boron thin film and the magnesium source contained inthe first protecting member and the second protecting member.

[0017] It is preferable that the substrate with the boron thin film is amonocrystalline sapphire substrate or a monochrystalline strontiumtitanate substrate. Use of these substrates can suppress unnecessaryreactions between the boron thin film and the substrate at hightemperatures. Preferably, the first protecting member is formed oftantalum or niobium and is filled with an inert gas. Filling the firstprotecting member with an inert gas is effective in preventing oxidationof the boron thin film and the magnesium source. It is preferable thatthe second protecting member is formed of quartz and its inside is in avacuum state. Evacuating the second protecting member can effectivelyprevent oxidation of the first protecting member by contact with theair.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above objects and advantages of the present invention willbecome more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

[0019]FIG. 1 shows the structure of a preferred embodiment of a pulsedlaser deposition (PLD) apparatus used in the formation of a boron thinfilm according to the present invention; and

[0020]FIG. 2 shows the structure of an apparatus for thermallyprocessing a superconducting magnesium diboride thin film according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0021] A method for fabricating a magnesium diboride (MgB₂) thin filmaccording to the present invention roughly involves two steps: Step 1 offorming a boron thin film as a precursor of magnesium diboride using aphysical deposition apparatus, and Step 2 of forming a superconductingmagnesium diboride thin film by diffusing magnesium into the boron thinfilm through reaction with magnesium.

[0022] In Step 1, the formation of the boron thin film can be achievedby pulsed laser deposition (PLD), sputtering deposition, electron beamevaporation, metallorganic chemical vapor deposition (MOCVD), chemicalvapor deposition, etc. The boron thin film formed by these methods canbe amorphous or crystalline. The characteristics of the boron thin filmvary slightly with the method applied to form the same.

[0023] Step 1 of forming a boron thin film by PLD will be described ingreater detail with reference to FIG. 1. A coin-like target 16 for usein the deposition of the boron thin film was prepared by stuffing acylindrical mold (having a diameter of 10-100 mm) with boron powderhaving a grain diameter of 1-5 μm and applying pressure on the order of5-10 tons. The target 16 is fixed to a support plate for the target 17and irradiated with an excimer laser beam. As a result, boron evaporatesfrom the target 16 and forms a boron thin film on a substrate 14 fixedto the top of a support plate 12 for substrate. In FIG. 1, referencenumeral 11 denotes a direction in which the laser beam is radiated, andreference numeral 15 denotes boron evaporation toward the substrate 14.

[0024] The boron deposition is carried out under the conditions of alaser pulse frequency of 1-10 Hz, preferably about 8 Hz, and a laserbeam energy density of 20-30J/cm² in consideration of boron's vaporizingtemperature. When boron deposition is continued for about 1-2 hoursunder the above conditions, an amorphous boron thin film having athickness of about 0.5-1 μm and a mirror-like glossy surface isobtained.

[0025] The substrate 14 on which the boron thin film is formed may be amonocrystalline sapphire (a variant from corundum (Al₂O₃)) substrate ora monocrystalline strontium titanate (SrTiO₃) substrate. This is becausethese substrates are chemically stable at high temperatures so thatreaction between substrate and thin film can effectively be suppressed.

[0026] In Step 2, a superconducting magnesium diboride thin film isformed by diffusing magnesium into the boron thin film through a thermalprocess to grow magnesium diboride crystal having uniform orientation.

[0027] Magnesium is easy to oxidize and has a melting temperature of650° C. and a vaporizing temperature of 1107° C., which are much lowerthan the melting point of 2100° C. and vaporizing temperature of 4000°C. of boron. Magnesium needs high-pressure reaction conditions due toits poor reactivity at atmospheric pressure. Magnesium also has highervapor pressure at a high temperature, and thus heating magnesium in asealed container can create a high-pressure environment. Based uponthese characteristics of magnesium, the boron thin film is reacted withmagnesium under continuous high-pressure. This process will be describedin greater detail with reference to FIG. 2.

[0028] Once a boron thin film 20 is formed on a substrate 21 as in Step1, the substrate 23 with the boron thin film 20 and a magnesium source22 are placed in a first protecting member 24 and then in a secondprotecting member 25. The magnesium source 22 may be provided in anyform, for example, powder, ribbon, or turning form, but the turning formis preferred because it has less surface area than the other forms sothat a chance of impurity contamination occurring is reduced.

[0029] Next, the second protecting member 25 is heated by a heat source26 and cooled, thereby resulting in a desired superconducting magnesiumboride thin film.

[0030] An example of the heat source 26, a horizontal type electricfurnace, is shown in FIG. 2.

[0031] It is preferable that the boron thin film 20 and the magnesiumsource 22 are heated at a temperature of 600-1000° C. If the heatingtemperature of the boron thin film 20 and the magnesium source 22 isless than 600° C., magnesium diffusion into the boron thin film 20hardly occurs. If the heating temperature exceeds 1,000° C., unintendedcrystalline structure is formed. The heat source 26 is not limited tothe type of FIG. 2, and a vertical or box type electric furnace can beused as the heat source 26.

[0032] Preferably, the thermal process is carried out in a short time.In particular, the temperature of the heat source 26 is raised to600-1000° C., and a sample is moved to a uniform-temperature centerregion of the heat source 26 within 30 minutes, preferably in 5 minutes.The sample is heated at the temperature of 600-1000° C. for 2 hours,preferably 30 minutes, drawn out of the heat source 26, and cooled for30 minutes to 2 hours, preferably 1 hour. Such a rapid thermal processcan effectively prevent degradation of the magnesium diboride thin filmwhich would be caused by chemical reaction with the substrate underlyingthe magnesium diboride thin film.

[0033] The first protecting member 24 is for preventing the boron thinfilm 20 and the magnesium source 22 from oxidizing, and thus it ispreferable that the first protecting member 24 is formed of a materialincapable of causing chemical reaction with the magnesium source 22 athigh temperatures. Suitable materials for the first protecting member 24include tantalum (Ta) and niobium (Nb). It is preferable that the firstprotecting member 24 is filled with an inert gas such as argon (Ar) toprevent oxidation of the boron thin film 20 and the magnesium source 22.In particular, magnesium changes into magnesium oxide by combinationwith oxygen present in the air. Thus, the sample should be reacted withmagnesium in the absence of oxygen to grow high-purity magnesium boridecrystals. The first protecting member 24 can be manufactured in anyshape without limitations. In an embodiment of the present invention, asthe first protecting member 24, a container made of Ta, morespecifically a Ta tube whose ends are sealed, is used.

[0034] The second protecting member 25 is for protecting the firstprotecting member 24 from oxidizing at high temperatures by contact withthe air, and it is not limited in shape. In an embodiment of the presentinvention, as the second protecting member 24, a container made ofquartz, and preferably a quartz tube whose both ends are sealed, isused. The inside of the second protecting member 25 is evacuated toprotect the first protecting member 24 from oxidizing in contact withthe air.

[0035] A result of an X-ray diffraction test on the superconductingmagnesium diboride thin film obtained by the method described aboveshows that the resultant superconducting magnesium diboride thin filmhas the c-axial orientation. In contrast, the magnesium diboride powderprepared by Nagamatsu et al. and the magnesium diboride wires formed byCanfield et al. are provn to be polycrystals grown in arbitrarydirections without orientation in a particular direction. The magnesiumdiboride thin film formed by the method according to the presentinvention has a superconducting critical temperature of 39 K and acritical current density of 8,000,000 A/cm². The superconductingcritical temperature of the magnesium diboride thin film according tothe present invention is the same as that of the conventionalsuperconducting magnesium diboride wires. However, the critical currentdensity of the magnesium diboride thin film according to the presentinvention sets the highest record of 20 times greater currenttransporting capability than the conventional superconducting wires.

[0036] The method for forming a superconducting magnesium diboride thinfilm according to the present invention enables formation of a magnesiumdiboride thin film with good superconductivity and crystalline c-axialorientation. The superconducting magnesium diboride thin film can beused in a variety of electronic devices employing superconducting thinfilms, such as precision medical diagnosis equipment usingsuperconducting quantum interface devices (SQUIDs) capable of sensingweak magnetic fields, microwave communications equipment used forsatellite communications, and Josephson devices. Computer systems with100 times greater computing speed can be implemented with thesuperconducting magnesium diboride thin film.

[0037] While this invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method for forming a superconducting magnesiumdiboride (MgB₂) thin film, the method comprising: (a) forming a boronthin film on a substrate; and (b) thermally processing the substrate onwhich the boron thin film is formed along with a magnesium source andcooling the resulting structure.
 2. The method of claim 1, wherein, instep (a), the boron thin film is formed by pulsed laser deposition,sputtering deposition, electron beam evaporation, metallorganic chemicalvapor deposition, or chemical vapor deposition.
 3. The method of claim1, wherein, in step (b), the substrate with the boron thin film and themagnesium source are heated at a temperature of 600-1000° C. in theabsence of air.
 4. The method of claim 1, wherein step (b) is carriedout in a state where the substrate with the boron thin film and themagnesium source are double sealed with a container made of tantalum orniobium inside and a container made of quartz outside.
 5. The method ofclaim 4, wherein both ends of the container made of tantalum or niobiumare sealed in an inert gas atmosphere, and both ends of the containermade of quartz are sealed in a vacuum.
 6. The method of claim 1,wherein, in step (b), a temperature of a heat source is raised to600-1000° C., and the substrate with the boron thin film and themagnesium source are placed inside the heat source, rapidly heated atthe temperature of 600-1000° C. for 10-60 minutes, and cooled.
 7. Themethod of claim 1, wherein the substrate on which the boron thin film isformed is a monocrystalline sapphire substrate or a monocrystallinestrontium titanate substrate.
 8. A superconducting magnesium diboridethin film formed by the method of claim 1 with the c-axial crystalorientation.
 9. The superconducting magnesium diboride thin film ofclaim 8, wherein the boron thin film in step (a) is formed by pulsedlaser deposition, sputtering deposition, electron beam evaporation,metallorganic chemical vapor deposition, or chemical vapor deposition.10. The superconducting magnesium diboride thin film of claim 8,wherein, in step (b), the substrate with the boron thin film and themagnesium source are heated at a temperature of 600-1000° C. in theabsence of air.
 11. The superconducting magnesium diboride thin film ofclaim 8, wherein step (b) is carried out in a state where the substratewith the boron thin film and the magnesium source are double sealed witha container made of tantalum or niobium inside and a container made ofquartz outside.
 12. The superconducting magnesium diboride thin film ofclaim 11, wherein both ends of the container made of tantalum or niobiumare sealed in an inert gas atmosphere, and both ends of the containermade of quartz are sealed in a vacuum.
 13. The superconducting magnesiumdiboride thin film of claim 8, wherein, in step (b), a temperature of aheat source is raised to 600-1000° C., and the substrate with the boronthin film and the magnesium source are placed inside the heat source,rapidly heated at the temperature of 600-1000° C. for 10-60 minutes, andcooled.
 14. The superconducting magnesium diboride thin film of claim 8,wherein the substrate on which the boron thin film is formed is amonocrystalline sapphire substrate or a monochrystalline strontiumtitanate substrate.
 15. An apparatus for fabricating a superconductingmagnesium diboride thin film, the apparatus comprising: a firstprotecting member receiving a substrate with a magnesium diboride thinfilm and a magnesium source for preventing the magnesium diboride thinfilm and the magnesium source from oxidizing in contact with the air; asecond protecting member receiving the first protecting member forpreventing oxidization of the first protecting member; and a heat sourcefor thermally processing the substrate with the boron thin film and themagnesium source contained in the first protecting member and the secondprotecting member.
 16. The apparatus of claim 15, wherein the substratewith the boron thin film is a monocrystalline sapphire substrate or amonocrystalline strontium titanate substrate.
 17. The apparatus of claim15, wherein the first protecting member is formed of tantalum or niobiumand is filled with an inert gas.
 18. The apparatus of claim 15, whereinthe second protecting member is formed of quartz and its inside is in avacuum state.
 19. The apparatus of claim 15, wherein both ends of thefirst protecting member are sealed in an inert gas atmosphere, and bothends of the second protecting member are sealed in a vacuum.
 20. Theapparatus of claim 15, wherein the heat source is a horizontal typeelectric furnace.